A Novel Mechanism of Sea-Surface-Microlayer Formation Driven by Terrestrial Runoff: A Source of Ice Nucleating Particles in Arctic Coastal Environments

Biological aerosols serve as efficient ice-nucleating particles (INPs), driving the liquid-to-ice transition in clouds and thereby shaping cloud thickness, lifetime, albedo, and precipitation. Although atmospheric INPs are known to derive from terrestrial, freshwater, and marine environments, their specific sources and microbial producers remain largely unresolved, especially in the rapidly warming Arctic. Here, we examine terrestrial runoff as a driver of INP enrichment during summer in Young Sound fjord, Northeast Greenland. INP concentrations active at -10 degC, were significantly elevated in the sea-surface microlayer (SML) and at river outlets relative to bulk seawater, correlating positively with freshwater input and negatively with salinity and distance from runoff. These patterns highlight terrestrial runoff as a major source of warm-temperature INPs. We further identify a novel mechanism of SML formation in Arctic coastal waters: direct accumulation of terrestrial particles and compounds at the surface due to density contrasts between fresh and seawater. Highly active biological INPs were also observed in the atmosphere (1.3-25.3 INP-10 m-3), with sequencing data suggesting terrestrial origins. Our findings demonstrate how terrestrial ecosystems enrich the coastal SML with biogenic INPs that are subsequently aerosolized, underscoring their importance for Arctic cloud processes under ongoing climate change.